Proliferative retinopathy (PR), which includes retinopathy of prematurity (ROP) and proliferative diabetic retinopathy (DR), represent a leading cause of blindness in both children and working-age adults. Although PR is characterized by pathological retinal vascular overproliferation or neovascularization, therapies aimed at specifically inhibiting production of vascular endothelial growth factor (VEGF) have failed to consistently improve clinical outcomes. This observation clearly indicates that additional molecular factors/pathways contribute to pathological retinal neovascularization in PR and may thus be viable therapeutic targets. In this regard, we became intrigued by our preliminary finding that strong indoleamine-2,3-dioxygenase (IDO) expression is present in the vitreous of patients with DR as well as in the endothelium of retinal neovessels from mouse pups undergoing oxygen-induced retinopathy (OIR). Further, in in vitro experiments, we found that up-regulation of IDO endows the vascular endothelial cells (ECs) with enhanced capacity for proliferation, migration and tube formation independently of VEGF. In light of these novel findings we hypothesize that that endothelial IDO possesses pro-angiogenic activity, and that targeted inhibition of this enzyme will block the pathological retinal angiogenesis/neovascularization in PR. To test this hypothesis, we will use a number of unique approaches, which include a stable IDO-overexpressing retinal vascular EC line and the Sleeping- Beauty (SB)-based nonviral gene integrating strategy capable of long-lasting human IDO (hIDO) transgene expression within retinal ECs in vivo. In Specific Aim 1, we will evaluate the pro-angiogenic activity of endothelial IDO in vitro and in vivo, with a focus on the the molecular mechanism(s) behind IDO-induced phenotypic switching of retinal ECs from a normal to a pro-angiogenic state. In Specific Aim 2, we will examine the therapeutic potential of pharmacological inhibition of retinal endothelial IDO in the prevention of experimental PR. In these studies, we seek to achieve the therapeutic goal using selective IDO inhibitor 1-metheyl-DL-tryptophan (1-mT), a small molecular chemical (molecular weight 218) that can be orally or intraperitoneally administered and is currently used in anti-cancer clinical trials with satisfactory safety profile. Taken together, this proposal addresses a critical scientific gap in the treatment of PR, and if proven effective, may be fast-tracked to a phase 1 clinical trial.